I sometimes get confused with scientific notation. Sometimes things are in italic font, sometimes in upright font. I know that it is $\ce{H2O}$, not $H_2O$. But is it $e$ or $\mathrm{e}$? Is it $pK_a$ or $\mathrm{pK_a}$ or something mixed?

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In chemistry, the universal scientific convention is:

  • Variables and physical constants are written in italic type.
  • Chemical elements, operators, units, descriptive subscripts, mathematical constants etc. are written in upright type.

Example: $\ce{H}$ stands for hydrogen, $H$ for enthalpy.

Examples: $H$ denotes enthalpy, but $H_\mathrm{m}$ denotes molar enthalpy ($\mathrm{m}$ is a mnemonic label for molar, and is therefore roman). $C_p$ and $C_V$ denote the heat capacity at constant pressure $p$ and volume $V$, respectively; but $C_{p,\mathrm{m}}$ and $C_{V,\mathrm{m}}$ denote the molar heat capacity at constant $p$ and $V$, respectively (note the roman $\mathrm{m}$ but italic $p$ and $V$).
[from: IUPAC 2-page summary]

Regarding the examples in the question:

  • italic $e$ stands for a variable, the elementary charge
  • upright $\mathrm{e}$ stands for an electron or Euler's number
  • It is $\mathrm{p}K_\mathrm{a}$, with the $\mathrm{p}$ operator and the $\mathrm{a}$ subscript for 'acid' being upright and $K$ being and italic variable. ($\mathrm{p}K_\mathrm{a}$)

References (with many more details and examples):

The convention is not as clear in other fields than chemistry. In physics, for instance, the APS style (writing most single-letter operators and abbreviations in italic) contradicts IUPAP and NIST .

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    $\begingroup$ The standardized symbol for molar heat capacity at constant pressure and at constant volume is $C_{\mathrm m,p}$ and $C_{\mathrm m,V}$, respectively (not $C_{p,\mathrm m}$ and $C_{V,\mathrm m}$ – this is an unfortunate nonconformity in the otherwise nice two-page summary). $\endgroup$ – Faded Giant Feb 16 '17 at 11:17

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